Ternary uranium alloys containing molybdenum with niobium or zirconium for use with nuclear reactors



United States Patent 3,343,947 TERNARY URANIUM ALLOYS CONTAINING MOLYBDENUM WITH NIOBIUM 0R ZIRCO- NIUM FOR USE WITH NUCLEAR REACTORS Carlo Fizzotti, Guglielmo Colabianchi, Benito Dalmastri, and Alberto Masperoni, Rome, Italy, assignors to Comitato Nazionale per IEnergia Nucleare, Rome. Italy No Drawing. Filed June 24, 1965, Ser. No. 466,834 Claims priority, application Italy, Jan. 29, 1965, 12,441/ 65, Patent 1,660 9 Claims. (Cl. 75-122.7)

This invention relates to uranium alloys. Particular alloys are herein described together with their heat treatments which have been developed by the inventors with the purpose of preventing in the fuel elements any preferential orientation, any coarse grains and any particular structure which during the research work proved to be undesirable; all these imperfections at the moment in which the fuel is employed in a nuclear reactor would result in dimensional modifications which are known under the name of growth under irradiation or would produce a rough surface of the element.

Molybdenum, added to uranium as an alloying element, is effective for producing ,a fine grains structure as a result of appropriate heat treatments; but, due to its large neutron absorbing cross section, it must be employed as an alloy component at the lowest possible percentage.

The inventors have discovered that by partially substituting the amount of molybdenum required to form a certain alloy, with niobium at percentages which will be specified hereinafter, the twofold result which is achieved is a good neutronic economy together with the specific advantages of an alloy containing the appropriate amount of molybdenum.

The inventors have also found that structures of uranium alloys with zirconium are not sufficiently thermally stable.

By adding a small percentage of molybdenum to them, as specified hereinafter, finer a grains and higher thermal stabilities are produced.

According to the invention, an alloy containing 0.1-:0.3% by weight of molybdenum and 0.6-:0.8% by weight of niobium or an alloy containing 0.5% by weight of molybdenum and 0.5% of niobium or an alloy containing 0.2-:0.4% by weight of molybdenum and 1.5+2.2% by weight of zirconium; the rest being in all three cases uranium with accidental impurities, 'shows a fine a grain structure after casting. The alloys after casting in the form of unrefined bars show grains the size of which ranges from 100 to 175 a.

A better refinement and more homogeneous grains can be obtained by heating again the alloy in the 7 phase (e.g. 900 C.) and subsequently cooling it at the rate of to 80 C. per minute. At a cooling rate lower than 5 C. per minute, rather coarse grains and lamellar structures are produced. At a cooling rate greater than 80 C. per minute, both a refinement of the grains and a finer lamellar structure are obtained; however the structures so produced are much less thermally stable (e.g. when subjected to a subsequent extended heating at 550 C.) than those obtained through a cooling rate ranging from 5 to 80 C. per minute.

The heat treatments of ternary alloy bar conforming to the above specifications give a structure with or. grains the size of which ranges from 20 to 100 depending on the cooling rate.

These uranium alloys are prepared by melting a uranium ingot, together with a basic alloy, in a graphite crucible to which a lining of electro-melted magnesium oxide has been applied by centrifugal casting.

The basic alloy is produced by melting uranium together with molybdenum and niobium or zirconium in 3,343,947 Patented Sept. 26, 1967 "ice an arc furnace and casting an ingot containing from 6 to 10% by weight of alloying elements.

The furnace charge for producing the uranium alloys of this invention is melted by means of high frequency induction heating in vacuum. The product is kept in the melted state inside the crucible at 1,300'1,400 C. for 30 minutes and then is cast in a graphite chill to which a lining of electro-melted magnesium oxide has been applied by centrifugal casting and thereafter it is left to cool in the chill.

By this way, uranium alloys bars are obtained the diameters of which range from 26 to 33 mm. and which are up to 1,000 mm. long.

The size of the a grains ranged from to g.

The size of the on grains in the unrefined cast bars can be modified within a fairly wide range, by means of a heat treatment consisting of heating the alloy up to a high temperature (e.g. 900 C.) and subsequently cooling it in a continuous way at the rate of 5 to 80 C. per minute. The treatment is carried out eiher in vacuum or in an inert atmosphere (as in argon) and, as already stated, it gives a structure of which the grain size ranges from 20 to 1001.1. depending on the cooling rate.

A bar which has undergone the above heat treatment is free from the preferential orientations and the remarkable distortions which generally occur in the case of a very sharp cooling as for instance in a quenching.

Some examples of heat treatments of the unrefined cast bar and the results achieved by them are hereinafter described.

(1) 30 mm. diameter bar, 1000 mm. long Percentage by weight of alloying elements:

(a) 0.2% by weight molybdenum and 0.7% by weight niobium (b) 0.5% by weight of molybdenum and 0.5 by

weight niobium (c) 0.3% by weight molybdenum and 2.0% by weight zirconium Heating in 7 phase (at 900 C.) during two hours in vacuum. Cooling at the rate of 60 C. per minute in vacuum. Average size of the a grains after heat treating:

M 40M (c) 20a Average interlamellar distance of the lamellar structure:

(a) 0.7 11. (b) 0.7 (c) 0.8

(2) 29 mm. diameter bar, 600 mm. long Percentage by weight of the alloying elements:

(a) 0.2% by weight of molybdenum and 0.7% by weight of niobium (b) 0.5% by weight molybdenum and 0.5% by weight of niobium (c) 0.3% by weight of molybdenum and 2.0% by weight of zirconium Heating in '7 phase (at 900 C.) during two hours in vacuum. Cooling at a rate of 6 C. per minute in vacuum. Average size of the a grains after heat treating:

M M (c) 35a Average interlamellar distance of the lamellar structure:

(a) 1.2;1. (b) 1.2 (c) 1.7a

What is claimed is:

1. Uranium-molybdenum-niobium alloys having a fine grain structure after casting, said alloys having a molybdenum content from 0.1 to 0.3% by Weight and a niobium content from 0.6 to 0.8% by weight, the remainder of said alloy being substantially composed of uranium.

2. A uranium-molybdenurn-niobium alloy having a fine grain structure after casting; said alloy having a molybdenum content of 0.5 by Weight and a niobium content of 0.5% by weight, the remainder of said alloy being substantially composed of uranium.

3. Uranium-molybdenum-zirconium alloys having a fine grain structure after casting, said alloys having a molybdenum content from 0.2 to 0.4% by weight and a zirconium content from 1.5 to 2.2% by weight, the remainder of said alloys being substantially composed of uranium.

4. Uranium-molybdenum-niobium alloys as described in claim 1 wherein the a grain size ranges from 100 1:0 175 prior to heat treatment.

5. Uranium-molybdenum-niobium alloys according to claim wherein the interlammellar distance of the lamellar structure is less than 1.7.

6. Uranium-molybdenum-zirconium alloys as described in claim 3, wherein the grains of said alloy are refined by means of a heat treatment comprising the steps of heating the alloy at a temperature of 900 C. and thence continuously cooling the same at the rate of 5 to 60 C. per minute.

7. Uranium-molybdenum-niobium alloys as described in claim 1, wherein the grains of said alloy are refined by means of a heat treatment which comprises heating the alloy at a temperature of about 900 C. and continuously cooling at the rate of 5-60" C. per minute.

8. Uranium-molybdenum-zirconium alloys as described in claim 6, wherein the grain size ranges from 100-175 microns prior to heat treatment.

9. Uranium-molybdenum-niobium alloys according to claim 7 wherein the on grain size ranges from 20 to IOU/.0 after the heat treatment, the size depending on the cooling rate.

References Cited UNITED STATES PATENTS 2,888,343 5/1959 McGeary et a1 -1227 FOREIGN PATENTS 1,381,990 11/1964 France. 1,152,267 8/1963 Germany.

784,882 10/ 1957 Great Britain. 809,597 2/1959 Great Britain. 983,803 2/ 1965 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

M. J. SCOLNICK, Assistant Examiner. 

1. URANIUM-MOLYBDENUM-NIOBIUM ALLOYS HAVING A FINE GRAIN STRUCTURE AFTER CASTING, SAID ALLOYS HAVING A MOLYBDENUM CONTENT FROM 0.1 TO 0.3% BY WEIGHT AND A NIOBIUM CONTENT FROM 0.6 TO 0.8% BY WEIGHT, THE REMAINDER OF SAID ALLOY BEING SUBSTANTIALLY COMPOSED OF URANIUM.
 3. URANIUM-MOLYBDENUM-ZIRCONIUM ALLOYS HAVING A FINE GRAIN STRUCTURE AFTER CASTING, SAID ALLOYS HAVING A MOLYBDENUM CONTENT FROM 0.2 TO 0.4% BY WEIGHT AND A ZIRCONIUM CONTENT FROM 1.5 TO 2.2% BY WEIGHT, THE REMAINDER OF SAID ALLOYS BEING SUBSTANTIALLY COMPOSED OF URANIUM. 